A typical photovoltaic (PV) device is a PV module that can convert photo-radiation into electrical current. A typical PV module includes two conductive electrodes sandwiching a series of semiconductor layers, which provide a junction at which the photo-conversion occurs.
Photovoltaic modules, which can be patterned into a plurality of PV cells, can be formed on an optically transparent substrate. This substrate can be any suitable, transparent substrate material such as glass (including, but not limited to soda-lime glass or float glass) or polymer (sheet or plates). A first of two conductive electrodes is provided over the transparent substrate. The first conductive electrode can be a transparent conductive oxide (TCO) layer (e.g., indium tin oxide). The TCO layer can be associated with a barrier layer between it and the transparent substrate and also with a buffer layer over the TCO layer, which together provide a TCO stack that functions as the first conductive electrode. Over the first conductive electrode (buffer layer if provided), a semiconductor layer is provided. The semiconductor layer can be a bi-layer that includes a semiconductor window layer (e.g., cadmium sulfide) and a semiconductor absorber layer (e.g., cadmium telluride). Over the semiconductor layer, there is a second conductive electrode (e.g., a back contact layer), an interlayer to seal the PV module, and a back cover to provide support for the PV module, in that order.
During operation, photons pass into the semiconductor layers and are absorbed at or near the junction between the window layer and the absorber layer. This produces photo-generated electron-hole pairs, the movement of which, promoted by a built-in electric field, produces electric current that can be output from the device.
PV module performance characteristics may be evaluated based on identifying the module's short circuit current (Isc), open circuit voltage (Voc), fill factor (FF), or open cell resistance (Roc) (collectively, variables). Short circuit current (Isc) is the current through a PV device when the voltage across the PV device is zero, i.e., when a solar cell is short circuited; it relates to the generation and collection of light-generated carriers and represents the largest current that can be drawn from the PV device. Open circuit voltage (Voc) is a value that determines the maximum voltage available from a PV device, which occurs at zero current; it is a measurement of the amount of recombination in the PV device. Fill factor (FF) is a value that determines the maximum power from a PV device and is defined as the ratio between the maximum power from the PV device and the product of Voc and Isc; a higher voltage provides a higher possible FF. Open cell resistance (Roc) is a value that determines the resistance provided from an open circuit PV device.
During manufacturing of PV modules, minor variations in process parameters may result in modules having dissimilar performance characteristics, measureable by testing and determining the above-identified performance variables. Dissimilar performance characteristics are undesirable because the design and performance of an array of PV modules may rely on each PV module performing according to product specifications. Therefore, it is desirable to manufacture PV modules that exhibit similar performance characteristics when installed in the field. Moreover, it is desirable to manufacture PV modules that maintain similar performance characteristics over the life expectancies of the PV modules. An efficient way to test and condition PV modules during and/or after manufacture is desired.